I like the watch command, but you may want to use the -n30 delay (or some other largish number) option with that to reduce the polling frequency, especially if you are pushing priority to the array rebuild. Hitting /proc/mdstat seems to be a bit heavy in terms of system calls.

One other thing I’d also suggest, is instead of running the resize on a live filesystem, bootup with a livecd such as finnix. That will allow you to run fsck directly before resize2fs, as newer versions seem to enforce. And to be working with an offline filesystem, rather than a live one.

Very nice writeup.